For one in five Americans, the presidential election can be summed up in two words: "old" John McCain versus "outsider" Barack Obama.

In a poll released today that asked people to say the first words on their minds to describe the candidates, McCain's age (19%) and Obama's calls for change (20%) were the most common responses.

The focus on the 25-year age gap between McCain and Obama gives an apparent advantage to the youthful Illinois senator. But the No 2 spontaneous answers provide good news for McCain: He was linked with "military service" by 9% of respondents while Obama was associated with "lack of experience" by 13%.

The candidates' biggest political liabilities loomed further down the list of words cited in the poll. The word "Bush" was linked to McCain by 7%, and the word "Muslim" was named for Obama by 3% - a reference to false rumours that the Democratic nominee has faked his Christianity.

The poll, conducted by Knowledge Networks for the Associated Press and Yahoo, has surveyed about 2,000 people since November to gauge how individuals' views are changing during the presidential campaign.

The repeated interviews show the candidates' images have evolved gradually since the fall, with both getting higher favourable and unfavourable marks as additional people form opinions.

Indeed, the poll illustrates the mercurial public images of both McCain, 71, and Obama, 46.

Four in 10 Obama supporters have the same opinion of him now that they did in November, with a slight majority of the remaining backers souring on him.

As for McCain, he is now described as "old" by people who did not match him up with a word in January, suggesting that US voters who are just beginning to follow the campaign are keenly aware of his age.

Both are seen favourably by about half of those surveyed, and unfavourably by about four in 10.

However, Obama's image has deteriorated with two crucial groups: 52% of whites view him negatively, up 12 points from November. And 48% of independents have an negative view of him, up from 31% last fall.

The poll of 1,759 adults was conducted from June 13-23 and had an overall margin of sampling error of plus or minus 2.3 percentage points. Included were interviews with 844 Democrats and 637 Republicans, for whom the margins of sampling error were plus or minus 3.4 points and 3.9 points, respectively.